To create an epidural anesthesia or analgesia, two basic techniques to introduce drugs in the spinal area of a patient may be used, namely; epidural and subdural or subarachnoid. In this kind of surgical procedures, an anesthetic is injected in the spinal cord and in the nerve roots to block the pain feelings in a body region, such as the abdomen, hips, legs, or pelvis during labor.
As said medical techniques implies some risks for the patients, it is important to consider the main parts of the human body involved in these procedures, such as the meninges and the cerebrospinal fluid, which protect the central nervous system. As it is well known, the meninges are comprised by three layers: the dura mater, the arachnoid and the pia mater. The dura mater is the outermost, strongest, non-flexible layer from those three; the arachnoid is the medium membrane and the pia mater is the innermost and fragile of the meninges layers. In turn, the cerebrospinal fluid is a clear bodily fluid occupying the subarachnoid space, which is the space between the arachnoid and pia mater layers in the meninges.
The insertion of a hollow spinal needle (e.g., a Tuohy needle) is used in the Epidural anesthesia until the epidural space has been reached, being the space located underneath the dura mater. Firstly, the area wherein the needle is inserted is blocked with local anesthesia. Then, the needle is inserted and removed after the catheter has passed through the epidural space, said catheter remains in this area. The anesthetic is injected in the catheter to block the body area either upwards or downwards the injection point, as needed. In case it is necessary to apply more anesthetic, the catheter is secured over the patient's back to be reused again.
However, as the dura mater and arachnoid layers are so close to each other, sometimes it is not possible to pierce the dura mater without piercing the arachnoid with the epidural anesthetic, then, in this kind of procedures, it is of the outmost importance for the surgeon to be very precise.
In the same way as above, the subdural or subarachnoid anesthesia is carried out except that the anesthetic is directly injected in the cerebrospinal fluid surrounding the spinal cord with the aid of a second spinal needle, such as a Whitacre needle, a Quinkle or a Sprotte needle, this needle being introduced into the first spinal needle (Tuohy needle). The subdural or subarachnoid anesthesia blocks the body part beneath the site wherein the anesthetic is delivered, or, above it, depending on the anesthetic dose and the technique used for its application. In other words, if the tip of the second needle is directed towards the body's upper part and the anesthetic is delivered therein then this body's part will be blocked and the same will occur when the anesthetic is delivered to the body's lower part. Sometimes, to carry out a continuous spinal anesthesia, instead of using the second needle a spinal catheter can be inserted and left in the site where the injection was made.
In fact any spinal needle will pierce the skin, the subcutaneous fat, the supraspinous ligament, ligaments, the epidural space (in the case of the epidural anesthesia), the dura mater and the arachnoid layers until the needle reaches the subarachnoid space wherein the spinal cord and the nerve roots are located, surrounded by the cerebrospinal fluid (in the case of the spinal anesthesia).
On the other hand, there is also the mix anesthesia consisting in the application of both the epidural and the subdural or subarachnoid anesthesia. The first is used in surgery to provide an anesthetic, particularly to deliver reinforcing doses, while the second is used to provide a longer term anesthesia to a patient.
It is very important to ensure that the injection will correctly pierce the desired area with the epidural and subdural or subarachnoid anesthesia, since otherwise the nervous, cardiovascular and respiratory systems may be affected with the anesthetic. Both the epidural and subdural or subarachnoid anesthesia may significantly affect the breathing, the heart beating and other vital functions. Moreover, there is a potential toxicity risk caused by high drug doses unnecessary to obtain a proper blocking. As mentioned above, in order to achieve the object of blocking the desired part of the body, the anesthetic flow direction dosed in the epidural or subarachonid space is very important.
In specific relation to the epidural anesthesia, techniques and devices have been developed in the prior art to locate the epidural space, being that known as “loss-of-resistance” the most common technique used in the prior art wherein a spinal needle such as a Tuhoy needle, is connected to a syringe wherein its plunger is initially offset to leave inside the syringe housing from 4 to 10 cubic centimeters of air. Then while the plunger stem end is pressed with the thumb to exert a slight pressure, the needle is introduced into the patient's body. In this manner, the reaching of the epidural space is indicated by a lower pressure than that existing within the needle, causing the syringe plunger to travel without resistance in the patient's direction.
However, the above technique has to be carried out very carefully, since as the dura mater and arachnoid layers are so close to each other as already mentioned, sometimes it is not possible to pierce the dura mater without piercing the arachnoid layer. Further, there is a risk of introducing an excess of air into the epidural space which may cause injuries to the patient.
A system to increase the visibility in the epidural space is disclosed in the U.S. Pat. No. 6,925,323. Said document discloses an epidural surgery method to enhance the visibility in a patient's epidural space, having the object of effectively carrying out the therapeutic surgery in said space. The method includes the steps of distending a patient's epidural space portion by filling said epidural space portion with a fluid supplied from a catheter; placing a sight hole in the epidural space distended portion by inserting the sight hole through the same catheter supplying the fluid making the distention, thereby achieving a visual image of the epidural space.
The U.S. Pat. No. 5,902,273 discloses a syringe capable of being pressured for the identification of the epidural space, this syringe is characterized by comprising a piston descending within a housing located in the needle upper part, visually indicating the surgeon the time when the needle has reached the epidural space. In the invention of this document, it is required that the surgeon places his/her finger in the distal end of the syringe to detect the pressure drop, which is visually checked with the movement of said piston.
The U.S. Pat. No. 6,773,417 shows an epidural space locating device comprising a body section having a first end and a second end; a channel extending between said ends wherein the first end may be coupled to a luer connector; and a collapsible rear chamber having one end coupled to the second end of the body section and the other end is exposed such that it allows exerting a pressure with one or more fingers of the hand such that the chamber keeps its shape when there is a positive pressure in it, and on the other hand, the chamber collapses when there is a negative or cero pressure in it indicating the location of the epidural space through a needle coupled to a luer connector; and the pressure loss within the chamber is detected by the fingers of the hand as the shape of the chamber collapses. Again, in the distal end of this device, a pressure has to be exerted.
Furthermore, it is worth mentioning the U.S. Pat. No. 7,175,608, wherein a device is provided to be placed between a syringe and the spinal needle; this device allows the visual detection of the time when the spinal needle has reached the epidural space by including a diaphragm which swollen when pressurized, and becoming depressurized when the needle has reached the epidural space.
As may be seen, the prior art devices still uses or are related to a syringe or a media wherein pressure has to be applied with the fingers, moreover, when the syringe and the spinal needle are coupled to each other, several centimeters in length can result, being a disadvantage in the management of the needle and the device once coupled since the weight may break the coupling, therefore, there is still a need for more precise, compact instruments and devices which allows the surgeon to detect the time when the epidural space has been reached.